In U.S. Pat. No. 4,005,927 and 4,251,130 and 4,372,645, gating circuits of the velocity matched type (VMG) are disclosed. In the cited patents, gating is produced by a traveling electrical modulating signal that propagates in synchronism with an optical signal. These are the so-called "traveling wave, velocity matched gates" (TW-VMG). In U.S. Pat. No. 4,372,645, the electrical circuit is tuned so as to produce a standing electrical wave. However, the two oppositely propagating waves that produce the standing wave are also velocity matched to the optical signal. This class of device is the so-called "standing wave, velocity matched gate" (SW-VMG).
The general problem with both classes of traveling wave gates resides in the fact that the refractive indices of the substrate material in which the gate is formed are very different at the two frequencies of interest, i.e., the electrical signal and optical signal frequencies. As a consequence, the electrical wavepath must be specially designed in order to satisfy the velocity match requirement, and to the extent that the velocities of the two signals are not carefully matched, the switching efficiency degenerates significantly.
In an alternative approach, described in 4,380,364, no attempt is made to satisfy the velocity match requirement. Instead, a velocity match is simulated in a basically velocity mismatch gate structure by shaping the electrical signal wavepath so that interaction of the electrical signal wave with the optical signal wave occurs only along selected regions of the electrical signal wavepath. More specifically, whenever the phase of the electrical signal (i.e., the electric field direction) is such as to interefere with the desired transfer of optical power between the coupled optical waveguides, the electrodes are physically displaced so that there is no longer any interaction between the electrical field and the optical signal.
The wavepath configuration disclosed in U.S. Pat. No. 4,372,643 was designed to match a standing wave modulating signal to a traveling wave optical signal. It is a problem with this device, however, that if the modulation signal losses are substantial, the locations of the nulls are displaced and vary as a function of time. As a consequence, the wave pattern is not uniform along the length of the device, thus complicating its design and fabrication. The design problem is further complicated if the ratio of the optical signal velocity and the modulating signal velocity is not an integer.